A lithium ion secondary battery using a nonaqueous electrolytic solution providing a high voltage is characterized by high energy density and widely used as power sources for mobile phones, notebook computers and others. Recently, with tightened CO2 regulation, use of a secondary battery in large-size products such as electric cars has drawn attention. In the circumstances, it has been desired to solve problems for improving safety and life and reducing cost.
As a positive electrode active material for a lithium ion secondary battery, LiCoO2 is well known. Since LiCoO2 has satisfactory characteristics, it is used in many lithium ion secondary batteries. However, Co as a raw material is expensive and a resource is uneven distributed. Like this, many variation factors are associated with LiCoO2 as problems. Particularly, when it is used in large-size products, price and constant supply of a resource are important for selecting a material. Thus, study of an alternate material is indispensable.
As another positive electrode active material, LiNiO2 is mentioned. Compared to Co, Ni is a raw material supplied from an abundant resource; but the price greatly varies due to demand balance. In LiNiO2, trivalent Ni is unstable and likely to change into divalent Ni, with the result that LiNiO2 changes into a non-stoichiometric composition. Furthermore, divalent Ni may possibly invade into a lithium site. For these reasons, it is difficult to control synthesis of LiNiO2. In addition, since LiNiO2 is thermally unstable, it is difficult to ensure safety of a secondary battery.
On the other hand, in view of cost and safety, a lithium manganese composite oxide, LiMn2O4, of a spinel type crystal structure having a three-dimensional lithium diffusion route is highly expected. A resource for Mn as a raw material for LiMn2O4 is abundant and relatively inexpensive. Besides these, Mn rarely causes thermal decomposition during overcharge and at a high temperature. Thus, Mn is advantageous in view of ensuring safety. However, LiMn2O4 causes e.g., deterioration with cycles and elution of Mn into an electrolytic solution if LiMn2O4 is stored at a high temperature. They are regarded as problems. These are caused by Jahn-Teller strain of trivalent Mn increased with Li insertion. As a result, the crystal structure is destabilized and performance deterioration and the like conceivably occur with cycles.
In order to reduce the Jahn-Teller strain, an attempt to substitute trivalent Mn with another element has been made. For example, Patent Literature 1 discloses that a capacity retention rate can be improved during overdischarge by using a lithium manganese composite oxide having a spinel structure and having a composition represented by formula LixMn(2-y)AlyO4 (0.85≦x≦1.15, 0.02≦y≦0.5), in which a part of Mn is substituted with Al, as a positive electrode active material. Furthermore, it has been confirmed that an effect of improving e.g., life is exerted by substitution with e.g., Mg and Ca (Patent Literature 2), Ti (Patent Literature 3) and Co, Ni, Fe, Cr (Patent Literature 4).
A lithium manganese composite oxide is a so-called 4 V-level positive electrode having a discharge potential of 4.2 V or less and a small discharge capacity. Thus, there is a technical problem in increasing energy density. As a method for improving the energy density of a lithium ion secondary battery, a method of increasing the action potential of a secondary battery is effective. Up to present, it is known to realize a 5 V-level action potential by substituting a part of Mn of LiMn2O4 with Ni, Co, Fe, Cu, Cr and the like (for example, Patent Literature 5, Non Patent Literature 1 and Non Patent Literature 2). These are called as a 5 V-level positive electrode.
Of them, a lithium manganese composite oxide in which a part of Mn site is substituted with Ni exhibits a flat discharge potential and a high capacity in the region of 4.5 V or more, and thus, expected as a high potential positive electrode active material. For example, in the case where a part of Mn site is substituted with Ni, Mn is present in a quadrivalent state. Discharge occurs by the reaction of Ni2+→Ni4+ instead of the reaction Mn3+→Mn4+. Since the reaction of Ni2+→Ni4+ provides a high potential of about 4.7 V, the lithium manganese composite oxide functions as a high-potential electrode material.
On the other hand, in various industrial fields including the automobile industry, it is expected to increase the demand for a lithium ion secondary battery. In the circumstance, a lithium manganese composite oxide using Fe as a substitution element is extremely advantageous in view of resource, environment and cost. In the case where a part of Mn site is substituted with Fe, the reaction of Fe3+→Fe4+ occurs in place of the reaction of Mn3+→Mn4+. The reaction of Fe3+→Fe4+ is known to occur at near 4.9 V. Up to present, a high potential spinel material in which a part of Mn site is substituted with Fe has been disclosed (Patent Literature 6 and Patent Literature 7).